EP1307956A1

EP1307956A1 - Method and device for localising single-pole earth faults

Info

Publication number: EP1307956A1
Application number: EP01965173A
Authority: EP
Inventors: Albert Leikermoser
Original assignee: Adaptive Regelsysteme GmbH
Current assignee: Adaptive Regelsysteme GmbH
Priority date: 2000-08-11
Filing date: 2001-08-06
Publication date: 2003-05-07
Anticipated expiration: 2021-08-06
Also published as: ATA14952000A; AT413447B; DE50106400D1; NO332685B1; HUP0300733A2; WO2002015358A1; AU2001285872A1; EP1307956B1; HU225928B1; NO20030650D0; NO20030650L

Abstract

The aim of the invention is to provide a method that can be simply implemented for the reliable localisation of single-pole earth faults in a branch or a line section of a neutral-point compensated or insulated electric supply network, in which a neutral point biasing voltage is measured between the neutral point of the supply transformer and earth, in addition to a zero current of each branch or line section of the network that should be monitored. The aim is primarily achieved as follows: the measured values of the neutral point biasing voltage and the zero currents are digitised at predetermined times and said measured values are stored in digital form in at least one electronic memory. The stored measured values from at least one measuring process before and at least one measuring process after the occurrence of a fault are evaluated using an appropriate mathematic procedure that localises single-pole earth faults.

Description

Method and device for locating single-pole earth faults

The invention describes a method and a device for executing the method for locating single-pole earth faults in a branch or a line section of a point-compensated or star-point-insulated electrical supply network in which a neutral point displacement voltage between the star point of the supply transformer and earth, or one proportional to the neutral point displacement voltage Measured variable, and a zero current, or a measured variable proportional to the zero current, of each branch or line section of the network to be monitored in a suitable manner.

Some methods for locating single-pole earth faults in medium and high voltage networks are known. Most of these methods use the ohmic component of the zero current, i.e. that component of the zero-sequence current vector in the direction of the neutral point displacement voltage, for locating earth faults, since in stationary, i.e. settled, system state the capacitive zero current components caused by the individual phase earth capacitances of the feeder can no longer be distinguished from the capacitive or inductive components of the fault current possibly contained in the zero current. However, these ohmic components are very small in relation to the capacitive and inductive components, which makes it difficult to reliably detect high-resistance earth faults due to the very small ohmic zero-current components.

Other methods use the phase relationship between zero current and neutral point displacement voltage to locate earth faults, such as EP 963 025. Here the zero currents of each branch to be monitored and the neutral point displacement voltage are measured and, based on the phase position of the measured quantities, it is determined whether a branch is faulty or is flawless.

The object of the invention is to create a method of the type mentioned at the beginning which can reliably locate earth faults in medium and high voltage networks using simple means.

This task is primarily solved for the method in that the measured values of the neutral point displacement voltage and the zero currents are digitized at predetermined times and the measured values are stored in digital form in at least one electronic memory and that the stored measured values from at least one measurement before and at least a measurement after an error occurs with a suitable mathematical method for locating single-pole earth faults. For The device achieves the object in that at least one analog / digital converter is provided, which converts the measured values into digital form at specific points in time, that at least one electronic memory is provided for storing the digitized measured values and that an evaluation unit is provided, which is suitable for carrying out a suitable mathematical method for locating single-pole earth faults on the basis of measured values from at least one measurement before and at least one measurement after an error occurs.

Because the measured values are available in digital form and saved, it is possible to analyze the transient settling behavior of the zero current and the star point displacement voltage after an earth fault has occurred. This method is able to separate the capacitive from the inductive components of the zero currents of the branches or line sections of the network and, due to this property, is capable of making statements about the faultiness in connection with single-pole earth faults on branches or line sections. Since these capacitive and inductive zero-current components are much larger than the ohmic zero-current components, this method allows a particularly sensitive and reliable location of earth faults.

It proves to be particularly advantageous if the functional relationship of the behavior of the zero current i ^ ^{k) of} each branch or to be monitored

Line section k of the network and the neutral point displacement voltage U _ne both for the case of freedom from errors, as well as for the case of errors by a linear differential equation or difference equation of second or higher order, or by their associated equivalent equation in an integral form, preferably in their numerical notation S _t (n) = a ^ S ^ n) - ^ a ₂ S ₂ {n) + a ₃ S ₃ (n) [+ ... + a _m S _m (n)]. Because the error case differs from the case of freedom from errors only by the coefficients of the differential equation, a faulty branch can be located very easily by determining these coefficients.

The method can be simplified by the neutral point displacement voltage U _ne and the zero currents i _g ^k) at equidistant times T, corresponding to a predetermined one

Sampling frequency f _A , with T = -, are measured and the sampling frequency f _A selected

YES becomes that it corresponds to an integer multiple of the network frequency f _N.

It is particularly advantageous for the further method to continuously use the instantaneous measured values, at the time nT, and the corresponding stored measured values to form an integer number j of previous network periods of the neutral point displacement voltage U _m and the zero currents i ^ ^k) a differential value for the zero current Δ / ^W (n) of each branch or line section of the network to be monitored and the neutral point displacement voltage Δw (n) and to store these difference values in digital form in electronic memories, since these difference values can then be made available for further use. The purpose of this difference is to subtract the stationary signal component from the transient signals generated in the event of an error before the error occurs.

A simplified method provides that the difference value for the zero-sequence current Ai ^k) (n) of each branch or line section of the network to be monitored and / or the difference value for the neutral point displacement voltage Δu (n) equal to the current one

Measured values of the zero currents i ^ ^k) or the neutral point displacement voltage U _ne at

Time nT be set. Although this reduces the accuracy of the method, this simplified method for locating high-resistance earth faults is less suitable. However, in the event of an earth fault, the stationary signal component is relatively small relative to the current measured value, so that subtraction of the stationary signal component can be avoided, especially for locating low or medium-resistance earth faults. When using this simplified method, it is no longer absolutely necessary to select the sampling frequency f _A as an integral multiple of the network frequency f _N. In the following, the treatment of differential values therefore includes both the method described above and the simplified method with the respective definition of the differential values.

If the absolute values of the instantaneous values of the difference values Δi ^(k) (n) of each branch or line section to be monitored and Δw (n) are compared with predefined trigger thresholds, a fault condition of the network can be recognized very easily if one or more of the absolute values of the difference values cause the associated trigger threshold is successively exceeded for a predefined number and / or the absolute amounts of the instantaneous values of the neutral point displacement voltage U _ne (n) successively exceed a predefined trigger threshold for a predefined number. It is particularly advantageous to indicate this error occurrence on an output unit, for example an output contact or an electronic display. The evaluation of the differential equation can be carried out very simply numerically if, from the stored difference values Δi ^m (n) and Δu (n) using any numerical integration method, for example the trapezoidal rule, the Simpson rule, or a numerical integration method of higher order, the value sequences Sι (n), S ^ n), S ₂ (n) and S ₃ (n), where the index n denotes the value of the respective value sequence at the time nT and the index 0 the time of the error occurrence, the four functions S ₍ (t ), 5 _j (t), S ₂ (t) and S ₃ (t) These calculated value sequences are usefully stored for further use.

The analysis of the fault status of the network can be carried out very easily with the help of this linear differential equation or second order difference equation by determining its constant coefficients ai, a ₂ and a ₃ , up to a _m if necessary, if the coefficients a-, a and a ₃ determined by any method, preferably by a method in the time domain, or in the image domain of any transformation, for example the Z transformation, such that the differential equation or difference equation or an equivalent equation in integral form for the functional relationship of zero current i ^ ^{k of} a monitored branch or line section k and the neutral point displacement voltage U _{ne is} fulfilled as best as possible. A particularly simple and quickly executable method is obtained if the coefficients ai, a ₂ and a ₃ are calculated for at least one branch or line section of the network to be monitored from a third-order linear system of equations, C _^ -a = b.

Particularly simple and quickly executable error detection criteria result from analysis of the coefficients in such a way that a monitored line section or branch is recognized as faulty if the associated coefficients a-ι and / or a _{2 are} negative or less than a predefined threshold and are recognized as fault-free becomes if the associated coefficients ai and / or a _{2 are} positive or greater than a predefined threshold and that a ₃ becomes zero in a star point isolated network. In addition, a switching action that is incorrectly detected as a fault can be recognized as connecting or disconnecting a branch or line section of the network if all measured values U _ne and i ^ ^{k) of} this branch or line section are essentially zero before or after the time of the switching action.

The end of the fault condition in the network can be recognized very easily if the effective value or absolute value of the neutral point displacement voltage U _{ne is} one falls below a predefined error-free threshold for a predetermined time and it is advantageous to continue the method after the end of the error state has been recognized.

A significant simplification of the method results if the digitized measured values of the neutral point displacement voltage U _ne and the zero currents i ₀ ^{(k) are} sequentially stored in one, preferably ring-shaped, electronic memory of length M, since certain measured values are then particularly easy again can be found. In addition, the ring-shaped memory stores only a limited number of measured values, which means that memory space can be saved. If, in addition, the length M of the annular memory is chosen such that it corresponds to an integral multiple of the quotient of the sampling frequency f _A and the network frequency f _N , the measured values of previous periods can be found particularly easily in the memory.

The further use of the calculated difference values is simplified if the calculated difference values Ai ^m (n) and Au (n) are stored sequentially in electronic memories of length N, preferably organized in a ring, and the length N is chosen to be less than or equal to the length M. , because then certain difference values are very easy to find again.

The evaluation of the measured values and the difference values is simplified if after the

Detection of an error state only exactly N difference values Ai ^(k) (n) and Au (n) of the neutral point displacement voltage U _ne and the zero currents i ^ ^{k) are} formed and stored and then the measured value acquisition and the formation of the difference value until the end of the detection Fault condition is interrupted. This ensures that the transient changes in the measured quantities are not overwritten after an error occurs, that is, they remain saved and thus an analysis of the error state is made possible.

If a microprocessor is used as the evaluation unit, a particularly flexible design variant is obtained. This makes it very easy to adapt or even change the method for locating earth faults.

The invention is described in more detail with reference to the following explanations with reference to the accompanying exemplary and non-restrictive drawings in FIGS. 1 and 2. In the drawing shows 1 shows the electrical equivalent circuit diagram of the zero-sequence system and a branch of a neutral point-compensated network,

Fig. 2 is a schematic representation of the measured value acquisition and evaluation.

1 shows the known electrical equivalent circuit diagram of a line section of a star point-compensated electrical supply network. Between the

Transformer star point and the ground point there is an erase coil 1, which is described by an ohmic conductance g _L and an inductance L. In the case of a star-insulated network, g _L and the reciprocal of L are zero. The leakage inductances and the ohmic resistances of the secondary winding of the supply transformer are represented by the longitudinal impedances Z _LT , which are assumed to be the same size for all three phases. The phase voltages C / _j , U ₂ and U _{3 are} fed into the network, here consisting of only one line section, and the phase currents i _γ , i ₂ and i ₃ flow. The phase earth voltages U _lE , U _2E and U _3E are present between the three phases and the earth. A line section, as is permissible for the considered frequency range of <100 Hz, is described by longitudinal line impedances Z _LL , consisting of an ohmic and an inductive term, and line derivative admittances Y _A1 , Y _A2 and Y _A3 , consisting of an ohmic and a capacitive term. The leakage currents i _Al, i _A2 and _A3 i flowing through the Leitungsableitadmittanzen Y _Al, Y _A2 and Y _A3. A fault location 8 is represented by an ohmic resistor R _F and the fault current i _{F flows} . The

Leakage currents i _A1 , i _A2 and i _A3 and the fault current i _p flow as zero current ι ^' "this

Line section in the star point compensated case back to the transformer star point via the quenching coil, while in the case of star point insulated mains operation the leakage currents have to flow back through the fault location. At the earthing point of the quenching coil, the zero currents of all existing line sections combine to form the sum of

Zero currents i _QS . On the consumer side, represented by consumer impedances Z _v , the consumer _currents i _vl , i _v2 and i _V3 flow.

This equivalent circuit diagram is used to derive relationships which are important for the method according to the invention. Regarding the following explanations, it should be noted in advance that the following formulas, unless otherwise stated, relate to the Laplace transforms of the respective electrical quantities, the occurring electrical quantities in order to simplify the However, the spelling cannot be written explicitly as a function of the complex frequency variable s, for example U _ne (s) is referred to as U _m .

As can be seen from the electrical equivalent circuit diagram in FIG. 1, the zero current i ₀ of any branch results as a very good approximation from the relationship

i ₀ = U "- Y + i _f + i _r ,

with the earth admittance Y = (Y _A1 + Y _A2 + Y _A3 ), the displacement current, or asymmetry current, i _v = (ü _l • AY _M + U ₂ ^■ AY _A2 + U ₃ ^■ AY _AZ ), the terms AY _Ai the asymmetry of the

Describe earth admittances and the fault current i _f in the event of a fault.

For the earth admittance Y of a branch or line section k, it suffices in the frequency range under consideration (<100 Hz) with sufficient accuracy Y by a

To represent parallel connection of the sum of the individual phase earth capacitances C and the sum of the ohmic conductance values g of the three phases against earth.

γm = _g w + C ^(k - s

For the sum of the zero currents of all branches i _{os, the} following then applies, provided that a fault current is only present in one of the branches,

^l 0S ⁼ * -> ne ^' J ^" * ^{" l} f + ^l vS'

where Y _{s is} the sum of the earth admittances of all branches and i _{vS is} the sum of the displacement currents of all branches. According to the relationship for the earth admittance Y of a branch, by summing, with the total conductance g _{s of} all branches and the total capacitance C _{s of} all branches,

Y _s = 8s + C _s > s.

The zero currents of all branches i _os flow back to the transformer star point via the quenching coil in neutral point compensated mains operation, whereby the following relationship can be established.

hs = -U _ne - Y _L Y _L is the admittance from the earthing point of the quenching coil to the transformer star point and is composed of an ohmic conductance g _L and the inductance L of the quenching coil. In the case of the star point-insulated network state, Y _L is zero

Y _L = 8 _L + *

L- s

By equating the above equations for the total zero current i _os , a relationship for the neutral point displacement voltage U is obtained.

If, by definition, the entire network is now error-free during the point in time marked with index (1) and the earth fault is identified by index (2), then the following apply, provided that the ohmic leads, the capacitances and the inductance of the quenching coil 1 during these two Measurement cycles have not changed and also the natural displacement currents of all branches have remained the same, taking into account the above derivations, the following relationships for the neutral point displacement voltage and the zero currents of a branch or line section of the network.

τj ^m = - -i-≤—

" ^e s + Y _L i ^ = ^ -Y + i _v

" _{M =} s + ψ l∞ = U∞ - Y + i, + t _f ^ne Y _S + Y _L

By elementally transforming these equations, a relationship is immediately obtained between the differences in the neutral point displacement voltages and the zero currents before and after a fault in any branch k.

or

Ai {s) = F (s) - Au (s)

The transfer function F (s) is through for a faulty branch a ₃

F {s]) = (y - ^~ Ys - - γ _L ^'a ι + a ₂ ^• S -rs with: a ₁ = 8 ^{m ~} - 8s - - 8 _L

«2 =: C ^(k) - -c _s

1 a ₃ = L

and for a faultless branch

With

«1 = 8 ^k) a ₂ =: C ^k)

given. By corresponding other assumptions and extensions of the model when deriving the above relationship, it is of course also possible to make differential or difference equations of higher orders, or in an equivalent integral form, S n) = a _l S ₁ (n) A- a ₂ S ₂ (n) + a ₃ S ₃ (n) A -... A- a _m S _m (n), and to carry out further evaluations using these equations.

As can be seen, the transfer function F (s) for the error-free case differs from the error case only by the coefficients a _v a ₂ and a ₃ . This makes it possible, based on the values of the coefficients a _λ , a ₂ and a _{3, to} make statements as to whether a section k of the network is error-free or faulty, since the following must apply directly.

a _v a ₂ <0 => the section k of the network under consideration is faulty, since at least those of the overall network are subtracted from the branch-specific conductance g ^(k) and the branch-specific capacitance C ^m .

a ₁ , a ₂ > 0 = the section k of the network under consideration is error-free since only the branch-specific conductance g ^w and the branch-specific capacitance C ^w occur.

a ₃ = 0 => the network is operated with neutral isolation.

The method according to the invention for locating a single-pole earth fault is therefore reduced to the determination of the coefficients aa ₂ and a ₃ from the system of equations

If the star point displacement voltage difference Au and the zero current difference Ai are known as functions of time, the three coefficients a _v a ₂ and a ₃ can now be determined with a suitable coefficient estimation method so that the equation error of the above equation transformed back into the time domain in the sense of the best approximation the sum of the equation errors is minimized over a number of samples in the quadratic mean. This task can also be solved by using methods in the image area, such as suitable parameter estimation methods based on the z-transformation or bilinear transformation. Of course, all of these methods can also be applied to integral forms of the above equations. In particular, a method for determining the coefficients aa ₂ and a _{3 is} presented here as a representative.

The relationship between the zero current differences and the neutral point displacement voltage differences or between the zero currents and the zero sequence voltage before and after the occurrence of an earth fault according to the above equation is first multiplied by the reciprocal of the complex frequency variable s and then transformed back into the time domain, which results in the following in the time domain Connection results.

With :

When using higher-order equations, corresponding further functions S ₄ (t), ..., S _m (t) and associated coefficients a ₄ , ..., a _{m result} , which must be evaluated accordingly in order to analyze the error state ,

The method according to the invention is described below with reference to the exemplary FIG. 2. Due to the numerical functioning of the method, it is necessary the time functions of the neutral point displacement voltage U _ne (t) and the zero current t ₀ (t) of each branch or line section of the network to be monitored are equidistant

Instants corresponding to a sampling time T = - at the sampling frequency f _A

Digitize YES and store it in a memory 7 in the form of measured value sequences U _ne (nT), or in a simpler notation U _ne (n), and i ₀ {nT), or in a simpler notation i ₀ (n). For this purpose, the neutral point displacement voltage U _ne (n), or a measured variable proportional to the neutral point displacement voltage, and the zero current i ₀ (n) of each branch or line section to be monitored are measured with a voltage measuring unit 2 or with current measuring units 3 and by means of analog / digital -Converters 4 digitized. The sampling frequency f _A is preferably chosen as an integer multiple of the network frequency f _N. Measured value differences can now be easily formed from these measured value sequences, with the stored measured value of an integer multiple of previous mains frequency periods always being subtracted from the current measured value, i.e. in the event of an error, the measured value before the error occurs from the measured value in the event of an error, in accordance with the relationships given above and the resultant measurement difference sequences Au (n) and Ai (n) are also stored.

For this purpose, it is expedient for the memories 7 to be organized as ring-shaped memories and for the length of the measured value memories M to correspond exactly to an integral multiple of the quotient of the sampling frequency f _A and the network frequency f _N , since it is then very simple to measure an integral multiple of a previous period find. This measurement value acquisition is carried out continuously until an earth fault 8 is detected. This occurrence of the fault can be recognized by a sudden change in Δw (t) and / or Δz ^" (t), since both variables must be approximately zero in the fault-free stationary state of the network. The method according to the invention can be used to detect low- or medium-resistance earth faults with the difference values Δw (t) and Δz ^' (t) also with the

Measured values of neutral point displacement voltage U _ne (n) and zero current i ₀ (n) can be carried out by yourself. The sampling frequency f _A no longer necessarily has to correspond to an integer multiple of the network frequency f _N.

When an error occurs, the measured value differences are compared with predefined trigger thresholds and an error is displayed on the display device 6 when a predetermined number of the absolute amounts of the differences successively exceed the associated trigger threshold and / or the absolute amounts of the instantaneous values of the Neutral point displacement voltage U _ne (n) successively exceed a predefined trigger threshold for a predefined number. After detection of an earth fault 8, exactly N differences of measured values are formed and stored, where N <M. From these N measured value differences, the associated integrals can be calculated in the evaluation unit 5 as a function of time by numerical integration, for example with the aid of the trapezoid or Simpson rule. It starts with the numerical integrations at time t = 0, i.e. when an error occurs. The mathematical problem is thus reduced to this with the help of the now known functions S ^ t), S ₂ (t), S ₃ (t) and S. (t), or their numerically determined value sequences S ^ n), S ₂ (n), S ₃ (n) and S, (ra), below

Using the differential equation or 2nd order difference equation in its integral form for each monitored branch or line section of the network to determine the as yet unknown coefficients a _x , a ₂ and a ₃ so that the equality of the left and right side of the above equation is determined by a predetermined number of samples is fulfilled as best as possible. It can be shown that the coefficients aa ₂ and a can be determined from a third-order linear system of equations, and the coefficients thus determined are best approximating with respect to minimizing the sum of the error squares of the equation errors. By solving the linear system of equations

C - a = b,

with a solution vector α consisting of the three coefficients a- _t , a ₂ and a ₃ , a 3x3 matrix C and a three-dimensional vector b, the elements of which are determined by the following relationships,

and

the three coefficients α α ₂ and ₃ can now be determined.

The solution of this 3rd order linear system of equations, for each monitored branch or line section of the network, can again be found by any known method and supplies the three coefficients ai, a ₂ and a ₃ which subsequently can be evaluated according to the general explanations above. In addition, switching operations in the network, that is to say the connection or disconnection of a branch or line section, can be determined very easily, since directly evident before or after a switching operation, all measured values before or after must be essentially zero. In this case, of course, no error status should be displayed.

After the location of the earth fault 8 or a switching operation, i.e. the assignment to a specific branch or line section of the network, in accordance with the associated coefficients ai, a ₂ and a ₃ , the end of the fault state is awaited and measurement data acquisition is started again. The end of the error state can be recognized by measuring and monitoring the neutral point displacement voltage, the error message being withdrawn if the neutral point displacement voltage falls below a predefined error-free threshold.

The description of the method according to the invention is merely exemplary and is in no way restrictive. In particular, all methods known to a person skilled in the art for determining the coefficients a ^ a ₂ and a ₃ can be used equivalently.

Due to the technical design of the quenching coil, it is possible that during the transient oscillation of the entire network after the occurrence of an earth fault in the quenching coil, magnetic saturation effects occur, which are not considered in the above mathematical model. As can be easily shown, the function S _j (t) or S _j (n) can be used to reconstruct the magnetic saturation state of the quenching coil, because due to the law of induction, a proportionality between the first derivative of the magnetic flux according to time and the coil voltage, i.e. the displacement voltage. Of course, this proportionality remains the same for the temporal integrals of the two quantities except for an additive constant. It can therefore be assumed that the change in magnetic flux from the time the error occurs is proportional to the change in the displacement voltage integral, i.e. approximately proportional to the function ^ (t) or S ^ n). If the approximation process, starting at zero time, i.e. when an error occurs, is only carried out up to the point in time at which the absolute value of the function S _l (t) or S _x (n) becomes greater than a predefined saturation threshold, this can be assumed that only the measured values of those process states are evaluated for which it is ensured that the quenching coil is not magnetically saturated and the mathematical model assumed is therefore correct. In addition, it is within the scope of the invention instead of local measurement value acquisition and measurement value evaluation for each line section, as shown in FIG. 2, to be arranged centrally at any point in the network, ie only one evaluation unit, a memory and a display are provided. The neutral point displacement voltage U _ne (t) is, for example, at the neutral point of the supply transformer against earth or through

Sum formation of the three phases - earth voltages measured and digitized fed to the evaluation unit. The zero currents are still measured for each line section and, for example, digitally transmitted to the evaluation unit. The evaluation of the state of each individual line section detected is then carried out in the evaluation unit using the method described above. Of course, there are also any

Combinations between local and central measured value acquisition and measured value evaluation acquired by the invention.

Furthermore, the arrangement and the number of A / D converters used are at the discretion of a person skilled in the art. In particular, it is also conceivable to use microcomputers with already integrated A / D converters.

Claims

claims

1. Method for locating single-pole earth faults in a branch or a line section of a star point-compensated or star point-insulated electrical supply network, in which a neutral point displacement voltage U _ne , or one of the measurement points proportional to the neutral point displacement voltage U _ne , and a zero current i ^ ^k) , respectively a measured variable proportional to the zero current i ₀ ^{(k) of} each branch or line section k of the network to be monitored can be measured in a suitable manner, characterized in that the measured values of the neutral point displacement voltage U _ne and the zero currents ijf * are predetermined

Points of time are digitized and the measured values are stored in digital form in at least one electronic memory and that the stored measured values from at least one measurement before and at least one measurement after an error occurs are evaluated using a suitable mathematical method for locating single-pole earth faults.

2. The method according to claim 1, characterized in that the functional relationship of the behavior of branch or line section zero current ^A) each branch or line section k to be monitored of the network and the neutral point displacement voltage U _ne both in the case of freedom from errors, as also in the event of a fault by means of a linear differential equation or difference equation of second or higher order, or by means of its associated equivalent equation in an integral form, preferably in its numerical notation,

S, (n) =, S _a (n) + a ₂ S ₂ (n) + « ₃ S ₃ (/») [+ ... + a _m S _m (n)] and that the fault occurs from the case of correctness only by coefficients ai, a ₂ and a ₃ , up to a _m if necessary.

3. The method according to claim 1 or 2, characterized in that the neutral point displacement voltage U _ne and the zero currents i ^ ^k) at equidistant times T,

1 can be measured according to a predetermined sampling frequency f _A , with T = -.

YES

4. The method according to claim 3, characterized in that the sampling frequency f _A is selected so that it corresponds to an integer multiple of the network frequency f _N.

5. The method according to claim 3 or 4, characterized in that continuously from the instantaneous measured values, at time nT, and the corresponding stored measured values of an integer number j of previous network periods, the neutral point displacement voltage U " _e and the zero currents i ^ ^k) relationships

A difference value for the zero current Ai ^(k) (n) of each branch or line section of the network to be monitored and the neutral point displacement voltage Au (n) is formed and these difference values are stored in digital form in electronic memories.

6. The method according to claim 5, characterized in that the absolute amounts of the instantaneous values of the difference values Ai ^m (n) of each branch or to be monitored

Line section and Au (n) are compared with predefined trigger thresholds and a fault condition is detected in the network if the associated trigger threshold is successively exceeded for a predefined number by one or more of the absolute values of the difference values and / or the absolute amounts of the instantaneous values of the neutral point displacement voltage U _ne (n) successively exceed a predefined trigger threshold for a predefined number and that this error occurrence is indicated on an output unit, for example an output contact or an electronic display.

7. The method according to claim 5 or 6, characterized in that from the stored difference values Ai ^ik) (n) and Au (n) using any numerical integration method, for example the trapezoidal rule, the Simpson rule, or a numerical integration method of higher order, the Sequences of values S, («), S ^ n), S ₂ (n) and

S ₃ (n), until possibly S _m (n), where the index n denotes the value of the respective sequence of values at the time nT and the index 0 the time at which the error occurred, of the functions S, (t), S _j (t), S ₂ (t) and S ₃ (t) according to the relationships be determined and saved.

8. The method according to any one of claims 2 to 7, characterized in that the

Coefficients a, a ₂ and a ₃ , up to a _m if necessary, can be determined by any method, preferably by a method in the time domain, or in the image domain of any transformation, for example the Z transformation, such that the differential equation or difference equation, or the equation equivalent to this in integral form for the functional relationship between zero current i ^ of a monitored branch or line section k and the neutral point displacement voltage U _ne for a predetermined number of samples is optimally fulfilled and that the values of the determined coefficients a ^ a ₂ and a ₃ , until a _m if appropriate, are used as error detection criteria for the corresponding monitored branch or line section.

9. The method according to claim 7, characterized in that for at least one branch or line section of the network to be monitored, the coefficients a, a ₂ and a ₃ from a third-order linear system of equations, Ca = b, with one of the three coefficients a- _\ , a ₂ and a ₃ existing solution vector a, a 3x3 matrix C and a three-dimensional vector b, whose elements from the calculated and stored value sequences S _; (rc), S ^ n), S ₂ (n) and S ₃ (n) can be determined by the following relationships, c _v = ∑S _k {n) -S _j {n) V * e {3} n = l and

& = Strength at ^yield. (ra) («) \ / ke {1,2,3}, and that the values of the coefficients ai, a ₂ and a ₃ thus determined as Error detection criteria can be used for the corresponding monitored branch or line section.

10. The method according to claim 8 or 9, characterized in that a monitored line section or branch is recognized as faulty if the associated coefficients ai and / or a _{2 are} negative or less than a predefined threshold and is recognized as error-free if the associated coefficients ai and / or a _{2 are} positive or greater than a predefined threshold and that a ₃ becomes zero in a star point isolated network.

11. The method according to claim 10, characterized in that the detected error state as a connection or disconnection of this branch or

Line section of the network is recognized when all measured values U " _e and i this

The branch or line section k is essentially zero before or after the time of the switching operation and that in this case no fault condition is displayed.

12. The method according to any one of claims 5 to 11, characterized in that the differential value for the zero current Ai ^(k) (n) of each branch and line section of the network to be monitored and / or the differential value for the neutral point displacement voltage Au {n) is the same the current measured values of the zero currents i or the neutral point displacement voltage U _ne at the time nT.

^ W ^ J ^ „. {0,1,2, ..., *}

Δu (n): = [U _πa (n) J

13. The method according to any one of claims 1 to 12, characterized in that the end of the fault condition in the network is detected when the effective value or absolute value of the neutral point displacement voltage U _m falls below a predefined error-free threshold for a predetermined time.

14. The method according to any one of claims 1 to 13, characterized in that the digitized measured values of the neutral point displacement voltage U _ne and the zero currents ι ₍ 0 are stored sequentially in each, preferably a ring-shaped, electronic memory of length M.

15. The method according to claim 14, characterized in that the length M of the annular memory is selected so that it corresponds to an integer multiple of the quotient of the sampling frequency f _A and the network frequency f _N.

16. The method according to claim 14 or 15, characterized in that the calculated difference values Ai ^(k) (n) and Au (n) are stored sequentially in, preferably ring-shaped, electronic memories of the length N and the length N is less than or equal to the length M is chosen.

17. The method according to claim 16, characterized in that after the detection of an error state, exactly N difference values A ^k) (n) and Au (n) of the neutral point displacement voltage U _ne and the zero currents _Q ^{k) are} formed and stored.

18. The method according to claim 16, characterized in that after the detection of an error state, exactly N difference values Ai (n) and Au (n) of the neutral point displacement voltage U _ne and the zero current i ^ ^{k) are} formed and stored and then the measured value acquisition and the formation of the difference value is interrupted until the end of the fault state is recognized and that the method is continued again after the end of the fault state has been recognized.

19. Device for locating single-pole earth faults in a branch or a line section of a star point-compensated or star point-insulated electrical supply network, consisting of a voltage measuring unit for the continuous measurement of the neutral point displacement voltage U _ne , or one of the measurement variables proportional to the neutral point displacement voltage U _ne , and for each of them Monitoring branch or line section k of the network of a current measuring unit for the continuous measurement of the respective zero current i, or a measurement variable proportional to the zero current i ^ ^k) , characterized in that at least one analog / digital converter is provided, which converts the measured values at specific times into a converts digital form that at least one electronic memory is provided for storing the digitized measured values and that an evaluation unit is provided which is used to carry out a suitable mathematical method for the location ng of single-pole earth faults based on measured values from at least one measurement before and at least one measurement after an error occurs.

20. The apparatus according to claim 19, characterized in that in the evaluation unit for each branch or line section k to be monitored of the network, the coefficients ai, a ₂ and a ₃ , up to a _m , if applicable, of a linear differential equation or second or higher order difference equation for the functional relationship between zero current i ^ ^{k) of} this branch or line section and the neutral point displacement voltage U _ne , or from their equivalent equation in integral form, preferably in numerical notation, S _t (n) = a ^ (n) + a ₂ S ₂ (n) + a ₃ S ₃ (/ ») [+ ... + a _m S _m (n)] by any method, preferably a numerical method, in the time domain or in the image domain of any transformation, for example the Z transformation , can be calculated and that the faulty branch or line section of the network can be determined using these coefficients.

21. The apparatus according to claim 19 or 20, characterized in that the

Voltage and current measuring units for performing measurements at equidistant

1 times T, with a predeterminable sampling frequency f _A and T - - are provided.

YES

22. The apparatus according to claim 21, characterized in that an integral multiple of the mains frequency f _{N can be} selected as the sampling frequency f _A in the voltage and current measuring units.

23. The device according to claim 21 or 22, characterized in that in the evaluation unit from the instantaneous measured values, at the time nT, and the corresponding stored measured values of an integer number j previous

Network periods of the neutral point displacement voltage U _ne and the zero currents i ^ ^k) according to the relationships

Differential values for the zero currents Ai ^(k) (n) of each branch or line section of the network to be monitored and the neutral point displacement voltage Au (n) can be calculated and that these differential values can be stored in digital form in at least one electronic memory.

24. The device according to claim 23, characterized in that an error state of the network can be identified by means of the evaluation unit and on a display unit. For example, an output contact or an electronic display can be displayed if the absolute amounts of the instantaneous values of the difference values Ai ^m (n) and Au (n) predefined

Trigger thresholds successively for a predefined number and / or additionally the absolute amounts of the instantaneous values of the neutral point displacement voltage U _ne (n) exceed a predefined trigger threshold in succession for a predefined number.

25. The device according to claim 23 or 24, characterized in that in the evaluation unit from the stored difference values Ai ^(k) {n) and Au {n) using any numerical integration method, for example the trapezoidal rule, the Simpson rule, or a numerical integration method higher order that

Sequences of values S _; (n), S _γ {n), S ₂ (n) and S ₃ (n), until possibly S _m (n), where the index n is the value of the respective value sequence at the time nT and the index 0 is the time of the error occurrence denotes, the functions S,. (t), S _j (t), S ₂ (t) and S ₃ (t) according to the relationships

are predictable and storable.

26. The apparatus according to claim 25, characterized in that in the

Evaluation unit for at least one branch or line section of the network to be monitored, the coefficients a ^ a ₂ and a ₃ from a third-order linear system of equations, C -a = b, with a solution vector a consisting of the three coefficients ai, a ₂ and a ₃ , a 3x3

Matrix C and a three-dimensional vector b, the elements of which are stored

Sequences of values S, (n), S ^ n), S ₂ (n) and S ₃ (n) can be determined by the following relationships,

= ∑S _k (n) S _j {n) V k, each {1,2,3} n = l and = ∑S, {n) S _k {n) V Λ e {1,2,3}, n = l can be calculated and that the faulty branch or line section of the network can be determined with these coefficients.

27. The apparatus of claim 20 or 26, characterized in that a branch or line section of the network can be identified as faulty in the evaluation unit if the associated coefficients ai and / or a _{2 are} negative or less than a predetermined threshold and can be identified as faultless, if the associated coefficients a- _\ and / or a _{2 are} positive or greater than a predetermined threshold.

28. The device according to claim 27, characterized in that a connection or disconnection of a branch or in the evaluation unit

Line section of the network is recognizable when all measured values U _ne and _Q ^k} this

Branch or line section before or after the time of the switching operation are essentially zero.

29. Device according to one of claims 23 to 28, characterized in that the difference value for the zero current Ai ^(k) (n) of each branch or to be monitored

Line section of the network and / or the difference value for the neutral point displacement voltage Au {n) is equal to the instantaneous measured values of the zero currents z ^ or the neutral point displacement voltage U _m at the time nT.

30. Device according to one of claims 19 to 29, characterized in that the end of the error state is recognizable in the evaluation unit when the effective value or the absolute value of the neutral point displacement voltage U _ne falls below a predefined error-free threshold.

31. Device according to one of claims 19 to 30, characterized in that for at least one variable to be measured, a preferably ring-shaped, electronic memory of length M is provided for sequential storage of the digitized measurement values and that the length M is an integer multiple the quotient of sampling frequency f _A and network frequency f _{N can be} selected.

32. Device according to one of claims 23 to 31, characterized in that for the difference values of at least one variable to be measured, a preferably annularly organized electronic memory of length N is provided for sequential storage of the difference values.

33. Device according to one of claims 19 to 32, characterized in that the evaluation unit is a microprocessor.